The present invention relates to an apparatus for visual inspection adapted to detect defects of a pattern to be inspected and more particularly, to a visual inspection apparatus used for inspecting pattern defects and foreign particles in the course of production of a semiconductor wafer, a photo-mask, a printed circuit board and so on.
In visual inspection of a semiconductor wafer, a method is known as disclosed in, for examples JP-A-2000-155099, according to which when an illumination beam of rays of light is reflected on the surface of a specimen and an optical image of the specimen is formed by interference of a zero-order diffraction ray and higher-order diffraction rays of a reflected beam, the contrast of the optical image is improved to detect a defect of a fine wiring pattern formed on the specimen with high sensitivity. A basic construction of an optical system used for the method is shown in FIG. 4. A beam of light emitted from a light source 8 reaches an aperture stop 11 through a concave mirror and lens 9 and then is rendered to be incident on a polarizing beam splitter 15 by way of a lens, a band-pass filter 12 and a field stop 13. The beam transmits through the polarizing beam splitter 15 and a resultant linearly polarized light beam leaving the splitter passes through a half waveplate 16 and a quarter waveplate 17 so as to be converted into an elliptically polarized light which in turn is irradiated on a specimen 1 by means of an objective lens 20. The direction of major axis of the elliptically polarized light can be controlled by rotating the quarter waveplate 17 and the ellipticity of the elliptically polarized light can be controlled by rotating the half waveplate 16. A beam of light reflected from the specimen 1 also enters the polarizing beam splitter 15 via the objective lens 20, quarter waveplate 17 and half waveplate 16 and then only an S-polarised component is reflected and led to an imaging optics comprised of an imaging lens 30 and a zoom lens 50. The optical image of the specimen 1 is formed on an image sensor 70. Then the image is converted to image data and defects are detected by processing the image data.
When, in this imaging optics, angles of the two waveplates are settled such that the specimen 1 is illuminated with a circularly polarized light, only a component having its polarization state unchanged during the reflection on the specimen surface is reflected at the polarizing beam splitter 15 and led to the imaging optics. On the other hand, when angles of the two waveplates are settled such that the specimen 1 is illuminated with an elliptically polarized light, part of a component having its polarization state changed during the reflection on the specimen 1 is also reflected at the polarizing beam splitter 15 and led to the imaging optics. Generally speaking, a beam of light rays diffracted by a linear pattern sometimes changes its polarization state but a zero-order ray does not change its polarization state. Accordingly, by illuminating the elliptically polarized light, the diffracted light component can be emphasized and then led to the imaging optics.
Hereinafter, the degree of emphasis of a diffracted beam attributable to the elliptically polarized light illumination will be called an SR intensity. The image sensor 70 is constructed of groups of pixels and typically, individual groups are called channels. For example, all the pixels on the sensor are divided into 32 or 64 channels.
The SR intensity has dependency on channels of the image sensor. Accordingly, under one illumination condition, the sensitivity differs between channels and therefore the condition is settled by a value of average sensitivity. In order to improve the inspection sensitivity, the difference in sensitivity between channels of the image sensor needs to be reduced.
An object of the present invention is to realize an optical system capable of improving the inspection sensitivity by reducing the dependency of the SR intensity upon channels on the image sensor.